/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A template class for tagged unions. */
#include <new>
#include <stdint.h>
#include "mozilla/Assertions.h"
#include "mozilla/Move.h"
#include "mozilla/OperatorNewExtensions.h"
#include "mozilla/TemplateLib.h"
#include "mozilla/TypeTraits.h"
#ifndef mozilla_Variant_h
#define mozilla_Variant_h
namespace IPC {
template <typename T>
struct ParamTraits;
} // namespace IPC
namespace mozilla {
template <typename... Ts>
class Variant;
namespace detail {
// Nth<N, types...>::Type is the Nth type (0-based) in the list of types Ts.
template <size_t N, typename... Ts>
struct Nth;
template <typename T, typename... Ts>
struct Nth<0, T, Ts...> {
using Type = T;
};
template <size_t N, typename T, typename... Ts>
struct Nth<N, T, Ts...> {
using Type = typename Nth<N - 1, Ts...>::Type;
};
/// SelectVariantTypeHelper is used in the implementation of SelectVariantType.
template <typename T, typename... Variants>
struct SelectVariantTypeHelper;
template <typename T>
struct SelectVariantTypeHelper<T> {
static constexpr size_t count = 0;
};
template <typename T, typename... Variants>
struct SelectVariantTypeHelper<T, T, Variants...> {
typedef T Type;
static constexpr size_t count =
1 + SelectVariantTypeHelper<T, Variants...>::count;
};
template <typename T, typename... Variants>
struct SelectVariantTypeHelper<T, const T, Variants...> {
typedef const T Type;
static constexpr size_t count =
1 + SelectVariantTypeHelper<T, Variants...>::count;
};
template <typename T, typename... Variants>
struct SelectVariantTypeHelper<T, const T&, Variants...> {
typedef const T& Type;
static constexpr size_t count =
1 + SelectVariantTypeHelper<T, Variants...>::count;
};
template <typename T, typename... Variants>
struct SelectVariantTypeHelper<T, T&&, Variants...> {
typedef T&& Type;
static constexpr size_t count =
1 + SelectVariantTypeHelper<T, Variants...>::count;
};
template <typename T, typename Head, typename... Variants>
struct SelectVariantTypeHelper<T, Head, Variants...>
: public SelectVariantTypeHelper<T, Variants...> {};
/**
* SelectVariantType takes a type T and a list of variant types Variants and
* yields a type Type, selected from Variants, that can store a value of type T
* or a reference to type T. If no such type was found, Type is not defined.
* SelectVariantType also has a `count` member that contains the total number of
* selectable types (which will be used to check that a requested type is not
* ambiguously present twice.)
*/
template <typename T, typename... Variants>
struct SelectVariantType
: public SelectVariantTypeHelper<
typename RemoveConst<typename RemoveReference<T>::Type>::Type,
Variants...> {};
// Compute a fast, compact type that can be used to hold integral values that
// distinctly map to every type in Ts.
template <typename... Ts>
struct VariantTag {
private:
static const size_t TypeCount = sizeof...(Ts);
public:
using Type = typename Conditional < TypeCount < 3, bool,
typename Conditional<
TypeCount<(1 << 8), uint_fast8_t,
size_t // stop caring past a certain point :-)
>::Type>::Type;
};
// TagHelper gets the given sentinel tag value for the given type T. This has to
// be split out from VariantImplementation because you can't nest a partial
// template specialization within a template class.
template <typename Tag, size_t N, typename T, typename U, typename Next,
bool isMatch>
struct TagHelper;
// In the case where T != U, we continue recursion.
template <typename Tag, size_t N, typename T, typename U, typename Next>
struct TagHelper<Tag, N, T, U, Next, false> {
static Tag tag() { return Next::template tag<U>(); }
};
// In the case where T == U, return the tag number.
template <typename Tag, size_t N, typename T, typename U, typename Next>
struct TagHelper<Tag, N, T, U, Next, true> {
static Tag tag() { return Tag(N); }
};
// The VariantImplementation template provides the guts of mozilla::Variant. We
// create a VariantImplementation for each T in Ts... which handles
// construction, destruction, etc for when the Variant's type is T. If the
// Variant's type isn't T, it punts the request on to the next
// VariantImplementation.
template <typename Tag, size_t N, typename... Ts>
struct VariantImplementation;
// The singly typed Variant / recursion base case.
template <typename Tag, size_t N, typename T>
struct VariantImplementation<Tag, N, T> {
template <typename U>
static Tag tag() {
static_assert(mozilla::IsSame<T, U>::value,
"mozilla::Variant: tag: bad type!");
return Tag(N);
}
template <typename Variant>
static void copyConstruct(void* aLhs, const Variant& aRhs) {
::new (KnownNotNull, aLhs) T(aRhs.template as<N>());
}
template <typename Variant>
static void moveConstruct(void* aLhs, Variant&& aRhs) {
::new (KnownNotNull, aLhs) T(aRhs.template extract<N>());
}
template <typename Variant>
static void destroy(Variant& aV) {
aV.template as<N>().~T();
}
template <typename Variant>
static bool equal(const Variant& aLhs, const Variant& aRhs) {
return aLhs.template as<N>() == aRhs.template as<N>();
}
template <typename Matcher, typename ConcreteVariant>
static auto match(Matcher&& aMatcher, ConcreteVariant& aV)
-> decltype(aMatcher.match(aV.template as<N>())) {
return aMatcher.match(aV.template as<N>());
}
};
// VariantImplementation for some variant type T.
template <typename Tag, size_t N, typename T, typename... Ts>
struct VariantImplementation<Tag, N, T, Ts...> {
// The next recursive VariantImplementation.
using Next = VariantImplementation<Tag, N + 1, Ts...>;
template <typename U>
static Tag tag() {
return TagHelper<Tag, N, T, U, Next, IsSame<T, U>::value>::tag();
}
template <typename Variant>
static void copyConstruct(void* aLhs, const Variant& aRhs) {
if (aRhs.template is<N>()) {
::new (KnownNotNull, aLhs) T(aRhs.template as<N>());
} else {
Next::copyConstruct(aLhs, aRhs);
}
}
template <typename Variant>
static void moveConstruct(void* aLhs, Variant&& aRhs) {
if (aRhs.template is<N>()) {
::new (KnownNotNull, aLhs) T(aRhs.template extract<N>());
} else {
Next::moveConstruct(aLhs, Move(aRhs));
}
}
template <typename Variant>
static void destroy(Variant& aV) {
if (aV.template is<N>()) {
aV.template as<N>().~T();
} else {
Next::destroy(aV);
}
}
template <typename Variant>
static bool equal(const Variant& aLhs, const Variant& aRhs) {
if (aLhs.template is<N>()) {
MOZ_ASSERT(aRhs.template is<N>());
return aLhs.template as<N>() == aRhs.template as<N>();
} else {
return Next::equal(aLhs, aRhs);
}
}
template <typename Matcher, typename ConcreteVariant>
static auto match(Matcher&& aMatcher, ConcreteVariant& aV)
-> decltype(aMatcher.match(aV.template as<N>())) {
if (aV.template is<N>()) {
return aMatcher.match(aV.template as<N>());
} else {
// If you're seeing compilation errors here like "no matching
// function for call to 'match'" then that means that the
// Matcher doesn't exhaust all variant types. There must exist a
// Matcher::match(T&) for every variant type T.
//
// If you're seeing compilation errors here like "cannot
// initialize return object of type <...> with an rvalue of type
// <...>" then that means that the Matcher::match(T&) overloads
// are returning different types. They must all return the same
// Matcher::ReturnType type.
return Next::match(aMatcher, aV);
}
}
};
/**
* AsVariantTemporary stores a value of type T to allow construction of a
* Variant value via type inference. Because T is copied and there's no
* guarantee that the copy can be elided, AsVariantTemporary is best used with
* primitive or very small types.
*/
template <typename T>
struct AsVariantTemporary {
explicit AsVariantTemporary(const T& aValue) : mValue(aValue) {}
template <typename U>
explicit AsVariantTemporary(U&& aValue) : mValue(Forward<U>(aValue)) {}
AsVariantTemporary(const AsVariantTemporary& aOther)
: mValue(aOther.mValue) {}
AsVariantTemporary(AsVariantTemporary&& aOther)
: mValue(Move(aOther.mValue)) {}
AsVariantTemporary() = delete;
void operator=(const AsVariantTemporary&) = delete;
void operator=(AsVariantTemporary&&) = delete;
typename RemoveConst<typename RemoveReference<T>::Type>::Type mValue;
};
} // namespace detail
// Used to unambiguously specify one of the Variant's type.
template <typename T>
struct VariantType {
using Type = T;
};
// Used to specify one of the Variant's type by index.
template <size_t N>
struct VariantIndex {
static constexpr size_t index = N;
};
/**
* # mozilla::Variant
*
* A variant / tagged union / heterogenous disjoint union / sum-type template
* class. Similar in concept to (but not derived from) `boost::variant`.
*
* Sometimes, you may wish to use a C union with non-POD types. However, this is
* forbidden in C++ because it is not clear which type in the union should have
* its constructor and destructor run on creation and deletion
* respectively. This is the problem that `mozilla::Variant` solves.
*
* ## Usage
*
* A `mozilla::Variant` instance is constructed (via move or copy) from one of
* its variant types (ignoring const and references). It does *not* support
* construction from subclasses of variant types or types that coerce to one of
* the variant types.
*
* Variant<char, uint32_t> v1('a');
* Variant<UniquePtr<A>, B, C> v2(MakeUnique<A>());
* Variant<bool, char> v3(VariantType<char>, 0); // disambiguation needed
* Variant<int, int> v4(VariantIndex<1>, 0); // 2nd int
*
* Because specifying the full type of a Variant value is often verbose,
* there are two easier ways to construct values:
*
* A. AsVariant() can be used to construct a Variant value using type inference
* in contexts such as expressions or when returning values from functions.
* Because AsVariant() must copy or move the value into a temporary and this
* cannot necessarily be elided by the compiler, it's mostly appropriate only
* for use with primitive or very small types.
*
* Variant<char, uint32_t> Foo() { return AsVariant('x'); }
* // ...
* Variant<char, uint32_t> v1 = Foo(); // v1 holds char('x').
*
* B. Brace-construction with VariantType or VariantIndex; this also allows
* in-place construction with any number of arguments.
*
* struct AB { AB(int, int){...} };
* static Variant<AB, bool> foo()
* {
* return {VariantIndex<0>{}, 1, 2};
* }
* // ...
* Variant<AB, bool> v0 = Foo(); // v0 holds AB(1,2).
*
* All access to the contained value goes through type-safe accessors.
* Either the stored type, or the type index may be provided.
*
* void
* Foo(Variant<A, B, C> v)
* {
* if (v.is<A>()) {
* A& ref = v.as<A>();
* ...
* } else (v.is<1>()) { // Instead of v.is<B>.
* ...
* } else {
* ...
* }
* }
*
* In some situation, a Variant may be constructed from templated types, in
* which case it is possible that the same type could be given multiple times by
* an external developer. Or seemingly-different types could be aliases.
* In this case, repeated types can only be accessed through their index, to
* prevent ambiguous access by type.
*
* // Bad!
* template <typename T>
* struct ResultOrError
* {
* Variant<T, int> m;
* ResultOrError() : m(int(0)) {} // Error '0' by default
* ResultOrError(const T& r) : m(r) {}
* bool IsResult() const { return m.is<T>(); }
* bool IsError() const { return m.is<int>(); }
* };
* // Now instantiante with the result being an int too:
* ResultOrError<int> myResult(123); // Fail!
* // In Variant<int, int>, which 'int' are we refering to, from inside
* // ResultOrError functions?
*
* // Good!
* template <typename T>
* struct ResultOrError
* {
* Variant<T, int> m;
* ResultOrError() : m(VariantIndex<1>{}, 0) {} // Error '0' by default
* ResultOrError(const T& r) : m(VariantIndex<0>{}, r) {}
* bool IsResult() const { return m.is<0>(); } // 0 -> T
* bool IsError() const { return m.is<1>(); } // 1 -> int
* };
* // Now instantiante with the result being an int too:
* ResultOrError<int> myResult(123); // It now works!
*
* Attempting to use the contained value as type `T1` when the `Variant`
* instance contains a value of type `T2` causes an assertion failure.
*
* A a;
* Variant<A, B, C> v(a);
* v.as<B>(); // <--- Assertion failure!
*
* Trying to use a `Variant<Ts...>` instance as some type `U` that is not a
* member of the set of `Ts...` is a compiler error.
*
* A a;
* Variant<A, B, C> v(a);
* v.as<SomeRandomType>(); // <--- Compiler error!
*
* Additionally, you can turn a `Variant` that `is<T>` into a `T` by moving it
* out of the containing `Variant` instance with the `extract<T>` method:
*
* Variant<UniquePtr<A>, B, C> v(MakeUnique<A>());
* auto ptr = v.extract<UniquePtr<A>>();
*
* Finally, you can exhaustively match on the contained variant and branch into
* different code paths depending on which type is contained. This is preferred
* to manually checking every variant type T with is<T>() because it provides
* compile-time checking that you handled every type, rather than runtime
* assertion failures.
*
* // Bad!
* char* foo(Variant<A, B, C, D>& v) {
* if (v.is<A>()) {
* return ...;
* } else if (v.is<B>()) {
* return ...;
* } else {
* return doSomething(v.as<C>()); // Forgot about case D!
* }
* }
*
* // Good!
* struct FooMatcher
* {
* // The return type of all matchers must be identical.
* char* match(A& a) { ... }
* char* match(B& b) { ... }
* char* match(C& c) { ... }
* char* match(D& d) { ... } // Compile-time error to forget D!
* }
* char* foo(Variant<A, B, C, D>& v) {
* return v.match(FooMatcher());
* }
*
* ## Examples
*
* A tree is either an empty leaf, or a node with a value and two children:
*
* struct Leaf { };
*
* template<typename T>
* struct Node
* {
* T value;
* Tree<T>* left;
* Tree<T>* right;
* };
*
* template<typename T>
* using Tree = Variant<Leaf, Node<T>>;
*
* A copy-on-write string is either a non-owning reference to some existing
* string, or an owning reference to our copy:
*
* class CopyOnWriteString
* {
* Variant<const char*, UniquePtr<char[]>> string;
*
* ...
* };
*
* Because Variant must be aligned suitable to hold any value stored within it,
* and because |alignas| requirements don't affect platform ABI with respect to
* how parameters are laid out in memory, Variant can't be used as the type of a
* function parameter. Pass Variant to functions by pointer or reference
* instead.
*/
template <typename... Ts>
class MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS MOZ_NON_PARAM Variant {
friend struct IPC::ParamTraits<mozilla::Variant<Ts...>>;
using Tag = typename detail::VariantTag<Ts...>::Type;
using Impl = detail::VariantImplementation<Tag, 0, Ts...>;
static constexpr size_t RawDataAlignment = tl::Max<alignof(Ts)...>::value;
static constexpr size_t RawDataSize = tl::Max<sizeof(Ts)...>::value;
// Raw storage for the contained variant value.
alignas(RawDataAlignment) unsigned char rawData[RawDataSize];
// Each type is given a unique tag value that lets us keep track of the
// contained variant value's type.
Tag tag;
// Some versions of GCC treat it as a -Wstrict-aliasing violation (ergo a
// -Werror compile error) to reinterpret_cast<> |rawData| to |T*|, even
// through |void*|. Placing the latter cast in these separate functions
// breaks the chain such that affected GCC versions no longer warn/error.
void* ptr() { return rawData; }
const void* ptr() const { return rawData; }
public:
/** Perfect forwarding construction for some variant type T. */
template <typename RefT,
// RefT captures both const& as well as && (as intended, to support
// perfect forwarding), so we have to remove those qualifiers here
// when ensuring that T is a variant of this type, and getting T's
// tag, etc.
typename T = typename detail::SelectVariantType<RefT, Ts...>::Type>
explicit Variant(RefT&& aT) : tag(Impl::template tag<T>()) {
static_assert(
detail::SelectVariantType<RefT, Ts...>::count == 1,
"Variant can only be selected by type if that type is unique");
::new (KnownNotNull, ptr()) T(Forward<RefT>(aT));
}
/**
* Perfect forwarding construction for some variant type T, by
* explicitly giving the type.
* This is necessary to construct from any number of arguments,
* or to convert from a type that is not in the Variant's type list.
*/
template <typename T, typename... Args>
MOZ_IMPLICIT Variant(const VariantType<T>&, Args&&... aTs)
: tag(Impl::template tag<T>()) {
::new (KnownNotNull, ptr()) T(Forward<Args>(aTs)...);
}
/**
* Perfect forwarding construction for some variant type T, by
* explicitly giving the type index.
* This is necessary to construct from any number of arguments,
* or to convert from a type that is not in the Variant's type list,
* or to construct a type that is present more than once in the Variant.
*/
template <size_t N, typename... Args>
MOZ_IMPLICIT Variant(const VariantIndex<N>&, Args&&... aTs) : tag(N) {
using T = typename detail::Nth<N, Ts...>::Type;
::new (KnownNotNull, ptr()) T(Forward<Args>(aTs)...);
}
/**
* Constructs this Variant from an AsVariantTemporary<T> such that T can be
* stored in one of the types allowable in this Variant. This is used in the
* implementation of AsVariant().
*/
template <typename RefT>
MOZ_IMPLICIT Variant(detail::AsVariantTemporary<RefT>&& aValue)
: tag(Impl::template tag<
typename detail::SelectVariantType<RefT, Ts...>::Type>()) {
using T = typename detail::SelectVariantType<RefT, Ts...>::Type;
static_assert(
detail::SelectVariantType<RefT, Ts...>::count == 1,
"Variant can only be selected by type if that type is unique");
::new (KnownNotNull, ptr()) T(Move(aValue.mValue));
}
/** Copy construction. */
Variant(const Variant& aRhs) : tag(aRhs.tag) {
Impl::copyConstruct(ptr(), aRhs);
}
/** Move construction. */
Variant(Variant&& aRhs) : tag(aRhs.tag) {
Impl::moveConstruct(ptr(), Move(aRhs));
}
/** Copy assignment. */
Variant& operator=(const Variant& aRhs) {
MOZ_ASSERT(&aRhs != this, "self-assign disallowed");
this->~Variant();
::new (KnownNotNull, this) Variant(aRhs);
return *this;
}
/** Move assignment. */
Variant& operator=(Variant&& aRhs) {
MOZ_ASSERT(&aRhs != this, "self-assign disallowed");
this->~Variant();
::new (KnownNotNull, this) Variant(Move(aRhs));
return *this;
}
/** Move assignment from AsVariant(). */
template <typename T>
Variant& operator=(detail::AsVariantTemporary<T>&& aValue) {
static_assert(
detail::SelectVariantType<T, Ts...>::count == 1,
"Variant can only be selected by type if that type is unique");
this->~Variant();
::new (KnownNotNull, this) Variant(Move(aValue));
return *this;
}
~Variant() { Impl::destroy(*this); }
/** Check which variant type is currently contained. */
template <typename T>
bool is() const {
static_assert(
detail::SelectVariantType<T, Ts...>::count == 1,
"provided a type not uniquely found in this Variant's type list");
return Impl::template tag<T>() == tag;
}
template <size_t N>
bool is() const {
static_assert(N < sizeof...(Ts),
"provided an index outside of this Variant's type list");
return N == size_t(tag);
}
/**
* Operator == overload that defers to the variant type's operator==
* implementation if the rhs is tagged as the same type as this one.
*/
bool operator==(const Variant& aRhs) const {
return tag == aRhs.tag && Impl::equal(*this, aRhs);
}
/**
* Operator != overload that defers to the negation of the variant type's
* operator== implementation if the rhs is tagged as the same type as this
* one.
*/
bool operator!=(const Variant& aRhs) const { return !(*this == aRhs); }
// Accessors for working with the contained variant value.
/** Mutable reference. */
template <typename T>
T& as() {
static_assert(
detail::SelectVariantType<T, Ts...>::count == 1,
"provided a type not uniquely found in this Variant's type list");
MOZ_RELEASE_ASSERT(is<T>());
return *static_cast<T*>(ptr());
}
template <size_t N>
typename detail::Nth<N, Ts...>::Type& as() {
static_assert(N < sizeof...(Ts),
"provided an index outside of this Variant's type list");
MOZ_RELEASE_ASSERT(is<N>());
return *static_cast<typename detail::Nth<N, Ts...>::Type*>(ptr());
}
/** Immutable const reference. */
template <typename T>
const T& as() const {
static_assert(detail::SelectVariantType<T, Ts...>::count == 1,
"provided a type not found in this Variant's type list");
MOZ_RELEASE_ASSERT(is<T>());
return *static_cast<const T*>(ptr());
}
template <size_t N>
const typename detail::Nth<N, Ts...>::Type& as() const {
static_assert(N < sizeof...(Ts),
"provided an index outside of this Variant's type list");
MOZ_RELEASE_ASSERT(is<N>());
return *static_cast<const typename detail::Nth<N, Ts...>::Type*>(ptr());
}
/**
* Extract the contained variant value from this container into a temporary
* value. On completion, the value in the variant will be in a
* safely-destructible state, as determined by the behavior of T's move
* constructor when provided the variant's internal value.
*/
template <typename T>
T extract() {
static_assert(
detail::SelectVariantType<T, Ts...>::count == 1,
"provided a type not uniquely found in this Variant's type list");
MOZ_ASSERT(is<T>());
return T(Move(as<T>()));
}
template <size_t N>
typename detail::Nth<N, Ts...>::Type extract() {
static_assert(N < sizeof...(Ts),
"provided an index outside of this Variant's type list");
MOZ_RELEASE_ASSERT(is<N>());
return typename detail::Nth<N, Ts...>::Type(Move(as<N>()));
}
// Exhaustive matching of all variant types on the contained value.
/** Match on an immutable const reference. */
template <typename Matcher>
auto match(Matcher&& aMatcher) const
-> decltype(Impl::match(aMatcher, *this)) {
return Impl::match(aMatcher, *this);
}
/** Match on a mutable non-const reference. */
template <typename Matcher>
auto match(Matcher&& aMatcher) -> decltype(Impl::match(aMatcher, *this)) {
return Impl::match(aMatcher, *this);
}
};
/*
* AsVariant() is used to construct a Variant<T,...> value containing the
* provided T value using type inference. It can be used to construct Variant
* values in expressions or return them from functions without specifying the
* entire Variant type.
*
* Because AsVariant() must copy or move the value into a temporary and this
* cannot necessarily be elided by the compiler, it's mostly appropriate only
* for use with primitive or very small types.
*
* AsVariant() returns a AsVariantTemporary value which is implicitly
* convertible to any Variant that can hold a value of type T.
*/
template <typename T>
detail::AsVariantTemporary<T> AsVariant(T&& aValue) {
return detail::AsVariantTemporary<T>(Forward<T>(aValue));
}
} // namespace mozilla
#endif /* mozilla_Variant_h */